Method of casting transparent films



Patented Jan. 14, 1936 PATENT OFFICE 2,027,829 METHOD OF CASTINGTRANSPARENT FILMS Edouard M. Kratz, Gary, Ind., assignor, by mesneassignments, to Marbo Products Corporation, Chicago, Ill., a corporationof Delaware N Drawing. Application November 8, 1933,

Serial No. 697,150

6 Claims. '(c1. 1s 57) This invention. relates to a method of castingtransparent films and more particularly to the use of a relieving orpartitioning agent in connection with a carrier Web to facilitate thestrip- 5 ping of the transparent sheet material from the carrier webafter the material has dried thereon.

In the casting of aqueous solutions of various film forming substances,such as casein, gelatin and the like,-I have found it mosts'uitable touse a flexible carrier web as the surface on which to cast the filmforming solutions. Practical types of carrier webs are described in myPatent No. 1,727,611 and in my pending application Serial No. 678,307,filed June 29,1933. It is desirable that the carrier web be 'plasticizedso as to impartto it the required degree of flexibility,

since otherwise there is a tendency of the surface of the carrier 'webto break down after continuous operation. In the absence of a,plasticizing agent in the carrier web, after continued operation, cracksform in the surface that are very objectionable.

Also, in the case of carrier webs which are coated with baked dryingoils, there is always a small quantity of free oil embodied in thecoating ma terial forming the web surface which has a tendency to'spewout of the casting surface when the carrier web is subjected to heatduring the drying operation. Although the quantity of oil that may bereleased to thesurface is very minute during any one cycle of the dryingoperation, after repeated continuous drying, the amount of oil whichspews to the surface is of sufficient quantity to cause-the dried film,formedby casting a solution onto. the carrier w'eb, to adhere to thesurface of the web.

Where the carrier web is provided with a surface of nitrocellulosecontaining castor oil as a plasticizing agent, there is likewise atendency for 40 the castor oil to work to the surface. There is also atendency of some of the materials which spew from the belt to forma filmon the surface of the carrier web that imparts a dull or smokyappearance to the finished sheet of cast material.

'Where the film solution being cast contains a fatty material, wax orthe like. in the form of an emulsion, there is a tendency of thesematerials to migrate or spew out of the semi-dried 5 sheet and beabsorbed by the carrier web. This a tendency for the outer surface tocase harden and the contraction of the film solution, which sults.having a melting point of from 118 to 122.F. and

is due to the fact that as the film dries, there is surface of thecarrier web. After a period of time, the carrier web may becomesaturated with one or more of these emulsified materials or plasticizingagents, with the result that when the carrier web is passed through adrying chamber, the heat causes the plasticizing agent or emulsifiedmaterial to spew out against the dried sheet and cause the sheet tostick to the carrier web.

It is therefore an object of this invention to provide a method ofcasting transparent film or sheet materials wherein the stripping of thesheet or film material from the carrier web is facilitated by the use ofa relieving or partitioning agent.'

. tion to provide an improved method of casting transparent sheet orfilm material onto flexible carrier webs, whereby the sheet or filmmaterial may be readily stripped from the surface of the carrier webwithout marring the polished surface of the carrier web or blemishingthe surface of the sheet or film material.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

The relieving or partitioning agent that'I prefer to apply to thesurface of the flexible carrier web is a substance that is inert bothtothe in- -greolients of the carrier web and also to the ingredients ofthe film that is being cast thereon. Among the inert substances that Ihave found to be satisfactory may be mentioned non-saponifiablesubstances of a greasy, unctuous character, such as solid or semi-solidhydrocarbons, like petroleum jelly and lanolin, or cholesterol,chlorinated diphenyls, and the like.

Petroleum jelly, however, gives the best re- For this purpose,a'whitepetroleum jelly a flash point of from 380 to 390 F. is preferred.In order to. assist in the spreading of the petroleum jelly over thesurface of the carrier web, the petroleum jelly is dissolved in acomparatively low boiling hydrocarbon, such as solvent naphtha.petroleum ether, or the like, so thatthe solution will dry out veryrapidly and leaveibehind a very thin film of petroleum jelly over thesurface of the carrier web.

v The peculiar property of naphtha of wetting out" and spreading rapidlymakes it very desirable as a solvent or diluting agent for the petroleumjelly. Other hydrocarbons, such as benzol, are not so well adapted forthis purpose because of the tendency ,of the petroleum jelly to coalesceand contract on the evaporation of these other hydrocarbons. Thus, whilevarious hydrocarbon solvents of relatively low boiling point may beused, most satisfactory results have thus far been obtained by the useof naphtha.

It might also be possible to use petroleum jellies having higher meltingpoints than above specified, but there is a tendency of the highermelting point petroleum jellies to crystallize and cause the surface ofthe finished film or sheet material to have a smoky or streakedappearance.

Further, it is not desirable to use a lower melting point jelly or oil,since there would be a tendency of the material to come to the surfaceduring the casting of the film solution, and the free oily hydrocarbonfloating upon the surface of the film solution would cause a break inthe surface tension Which would result in holes or pores in the finishedfilm.

Various concentrations of petroleum jelly and naphtha may be employed,but preferably a concentration of from to of 1% of petroleum jelly byweight of the solution is employed. The concentration of petroleum jellyin the naphtha solution may be increased or decreased, according to theamount of adhesion desired between the dried film: or sheet material andthe carrier web.

An excessive amount of petroleum jelly applied to the surface of thecarrier web is not desirable, since there is a tendency of the driedfilm to break loose from the carrier web at some stage prior to thefinal drying operation. This makes it more difficult to wind the driedfilm or sheet material on a mechanical winding device.

Furthermore, it is not desirable to have an insufficient amount ofpetroleum jelly, owing to the tendency to tear the film or sheetmaterial if there is too great an adhesion between the dried film orsheet material and the carrier web. Consequently, the quantity ofpetroleum jelly in solution in the naphtha can be adjusted to any degreeso as to give the proper relief between the dried film or sheet materialon the carrier web.

It has been found very difiicult to apply a film of petroleum jelly ofthe desired thickness in continuity to the surface of a carrier web bysuch mechanical devices as rollers, buffers or other coating apparatus.The solution of petroleum jelly in naphtha can, however, be readilyapplied to the casting surface of the carrier web by a felt wick, whichis permitted to lightly contact the surface of the carrier web. A softfelt wick is employed so as not to scratch ormar the polished surface ofthe carrier web. The lower end of the wick is kept immersed in thesolution of petroleum jelly in naphtha and capillary action is reliedupon to bring the solution into contact with the surface of the carrierweb. Because of the high degree of volatility of naphtha.

. the solvent evaporates rapidlyand leaves a thin,

continuous and uniform coating of petroleum jelly on the casting surfaceof the can'ier web.

Inasmuch as petroleum jelly does not crystallize, it has a tendency toflow out with the naphthe. and form a thin film that does not form anystreaks or cloudy effect upon the surface of the carrier web. Also,since the petroleum jelly is inert toward the ingredients of the carrierweb or of the film or sheet forming solution, the thin film of petroleumjelly protects the surfaces of both the film or sheet material and ofthe carrier web.

Where the casting of transparent sheet or fihn material is carried outas a continuous operation, as disclosed in my Reissue Patent No. 17,854,the solution of petroleum jelly in naphtha is continuously applied tothe bare surface of the carrier web after the sheet material has beendried and stripped therefrom and prior to the casting operation. Thefilm of petroleum jelly is thus renewed during each cycle of the film orsheet forming operation.

The thin, transparent sheet or film material herein referred may be suchas is described in my Patents Nos. 1,893,172, 1,866,410, 1,847,656 andvarious copending applications. I do not, however, restrict .the presentinvention to any particular film or sheet forming composition, since myinvention has wide applicability in this general field.

The particular advantages of my invention nevertheless are realized inconnection with the casting of aqueous solutions of film formingmaterials on flexible carrier webs. Solutions of this type generallyinclude dispersing or emulsifying agents in the nature of soap forlowering the surface tension of the solution and thus aiding in thespreading of the solution on the carrier surface. The presence of suchdispersing or emulsifying agents serves an even more important functionalong this same line where the sheet forming solution is cast upon asurface having a coating of petroleum jelly, in that they tend to reducethe surface tension and thereby facilitate the spreading of the solutionover the greasy surface of the carrier web. This enables the sheetforming solution to be cast into films of uniform thickness and freefrom pin holes or other blemishes.

As the sheet or film is stripped from the surface of the carrier web, itnaturally picks up a minute quantity of the petroleum jelly on itssurface. The quantity of petroleum jelly thus adhering to the surface ofthe film or sheet material is not sufficient to impair the transparencyof the film and is not, in fact, visible to the naked eye. The almostmicroscopic film of petroleum jelly on the surface of the sheet materialnevertheless serves a useful function in imparting D "slip" to the sheetmaterial. By this is meant the property of the sheet material, whenstacked, to be removed sheet by sheet without sticking. It frequentlyhappens that sheets of casein or gelatin composition will not slipreadily, either owing to surface characteristics or to the fact that thesheets are charged with static electricity.

I The presence of even a minute trace of petroleum jelly on the surfacesof the sheet material largely eliminates this tendency of the sheets tostick together and gives them the desired degree of slip.

Where the sheet material is being made in a continuous operation, it isobviously necessary to renew the film of petroleum jelly on the carrierweb. This is satisfactorily accomplished by permitting the wick,referred to above, to remain inconstant contact with the bare surface ofthe carrier web as it passes on into the coating or casting machine.Under some circumstances, however, the application of the petroleumjelly to the surface of the carrier web may be interrupted from time totime, as where the amount of petroleum jelly remaining on the web afterthe stripping operation is still sufficient and does not requirerenewing.

l am aware that numerous details of the process may be varied through awide range without departing from the principles of this invention, andI, therefore; do not purpose limiting the patent'granted hereonotherwise than necessitated by the prior art. I

i I claim as. my invention: a

l. The method of protecting the surface of a carrier web in the castingof transparent films thereon, which comprises coating said surfacewithasolution of petroleum jelly in a volatile coated surface.

3. In the casting of transparent films on a flexible carrier web, thestep of protecting the surface of said carrier web by'applying thereto atemporary thin coating of a solution of white petroleum jelly in naphthaand evaporating the naphtha.

4. In a continuous procesatorthe. casting of e transparent sheet or filmma on an endless flexible carrier web, the step oil continuouslyapplying to the bare casting surface or said carrier web a solution ofpetroleum jelly in a volatile hydrocarbon solvent.

5. In a continuous process for the casting of transparent sheet or filmmaterial on an endless flexible carrier web, the step of continuouslyapplying to the bare casting surface of said carrier web a solution ofpetroleum jelly in naphtha in whichthe petroleum jelly is present in anamount equal to between 9.5 and 0.75% by weight of the solution. M

l 6. The method oi! continuously forming transparent sheets or filmswhich comprises wicking tha onto a casting surface, allowing the naphthasolution to'spread out on said surtace to fully cover the same,evaporating oif thenaphtha to leave .a thin uniform coating of petroleumjelly on thesuriace, casting an aqueous film forming solution on thejelly coated surface, drying the solution thereon, stripping theresulting film therefrom and repeating said casting and dryingoperations.

. v .15 a solution "of petroleum jelly dissolved in napha 25 EDOUARDM.KRA'I'Z.

